This invention relates to an automatic sample injector for injecting a liquid sample into the vaporization chamber of a gas chromatograph. In particular, this invention relates to an improved method of operating such an automatic sample injector.
A syringe is commonly used for having a liquid sample vaporized for analysis by means of a gas chromatograph. Such a syringe comprises a barrel, a plunger adapted to slide inside the barrel in a liquid-tight relationship therewith, and a needle at the tip of the barrel, having a liquid passage therethrough. After a liquid sample is introduced into the barrel, the needle is caused to penetrate a septum (a rubber membrane) and is inserted into the vaporization chamber such that the liquid sample can be dispersed. The liquid sample which has been atomized is vaporized by heat and is transported into the column by a carrier gas introduced into the vaporization chamber.
An automatic sample injector is for carrying out these processes automatically and comprises a syringe-driving mechanism for moving the syringe with respect to the vaporization chamber and inserting the needle into the vaporization chamber and a plunger-driving mechanism for moving the plunger with respect to the barrel for sucking in a liquid sample into the barrel or injecting it out of the barrel.
Roughly speaking, the plunger-driving mechanisms can be divided into two kinds, one being by an open loop control by means of a stepping motor and the other being by a closed loop servo control by means of a DC motor. According to the former, the opening and closing of the switches for each magnetic pole of the stepping motor are controlled such that the rotor can be rotated by angles determined by the distribution of the magnetic poles. The advantage of this method is that the circuit is of a simple structure and is easy to control because it is an open loop control. For this reason, many prior art automatic sample injectors made use of this kind of mechanism.
With prior art driving mechanisms, however, rotary motion of the rotor of the stepping motor was accompanied by pulsation (or so-called "cogging"), and it was difficult to move the plunger smoothly at a constant speed. As a result, gas foams were likely to be generated when a sample is sucked in, adversely affecting the accuracy in the amount of the sample to be sucked in.